The protozoan parasite Toxoplasma gondii, a biodefense category B pathogen, causes life-threatening disease in immunocompromised individuals. Protective immunity against systemic T. gondii infection (as well as gut immunopathology after oral infection) is dependent on the induction of a potent Th1-polarized immune response, something that it is itself dependent on robust T. gondii-driven dendritic cells (DC) production of IL-12. Much experimental work has gone into defining the molecular mechanisms responsible for T. gondii-induced production of IL-12 by CD8+ splenic DCs. We have biochemical evidence for the formation of a T. gondii antigen-driven lipid raft recognition complex composed of TLR5, TLR11, C5aR and CCR5. Robust IL-12 production by splenic DCs is compromised by the absence of any of the receptors in the complex. Further, genetic deficiency of each of these receptors is associated with compromised host resistance to systemic infection with T. gondii. Our preliminary data indicate that JNK is the convergence point for the integration of both CCR5-/C5aR- and TLR-triggered pathways of T. gondii-induced splenic DC IL-12 production, and that the JNK scaffold protein, JIP1, is required for optimal IL-12 induction by such cells. While mucosal DCs are activated to produce IL-12 by T. gondii antigens, knowledge of innate recognition of T. gondii by mucosal DCs lags-the critical DC subset and relevant receptors remain to be identified. This proposal is based on the hypotheses that signaling through a recognition complex containing CCR5, C5aR and TLRs is critical for T. gondii-driven splenic DC IL- 12 production;and that this same receptor complex drives biologically-relevant IL-12 production by CD8+ mucosal DCs. The studies in this proposal will characterize the molecular mechanisms underlying T. gondii-triggered IL-12 production by splenic and mucosal DCs using both in vitro and in vivo approaches, the latter to define the relevance of in vitro-defined mechanisms to host defense an immunopathology. PUBLIC HEALTH RELEVANCE: The protozoan parasite Toxoplasma gondii, a category B biodefense pathogen, causes life-threatening disease in immunocompromised individuals. While effective at controlling acute disease, available drugs fail to clear chronic, latent infection with T. gondii. An in-depth understanding of the molecular mechanisms by which the immune system is normally able to control T. gondii is thus essential for the development of novel preventive and therapeutic strategies for this important parasite. Here, we propose to study the molecular events that take place during the initial interaction of host cells and pathogen. Those signals are known to be essential for the immune system become aware of the presence of the noxious microbe and initiate protective immunity.